1. Field of Invention
The invention relates, generally, to a method for determining an injection law of a fuel injector (i.e., for determining a law that binds actuation time to quantity of injected fuel) and, particularly, for determining such law using a roller-test bench.
2. Description of Related Art
An electromagnetic fuel injector (e.g., of the type described in Patent Application EP1619384A2) includes a cylindrical tubular body having a central feeding channel, which performs the fuel-conveying function, and ends with an injection nozzle regulated by an injection valve controlled by an electromagnetic actuator. The injection valve is provided with a pin, which is rigidly connected to a mobile keeper of the electromagnetic actuator to be displaced by the action of the electromagnetic actuator between a closing position and an opening position of the injection nozzle against the bias of a closing spring, which pushes the pin into the closing position. The valve seat is defined by a sealing element, which is disc-shaped, inferiorly and fluid-tightly closes the central duct of the supporting body and is crossed by the injection nozzle. The electromagnetic actuator includes a coil, which is arranged externally about the tubular body, and a fixed magnetic pole, which is made of ferromagnetic material and is arranged within the tubular body to magnetically attract the mobile keeper.
Normally, the injection valve is closed by effect of the closing spring that pushes the pin into the closing position in which the pin presses against a valve seat of the injection valve and the mobile keeper is distanced from the fixed magnetic pole. To open the injection valve (i.e., to move the pin from the closing position to the opening position), the coil of the electromagnetic actuator is energized so as to generate a magnetic field that attracts the mobile keeper towards the fixed magnetic pole against the elastic bias exerted by the closing spring (during the opening step, the stroke of the mobile keeper stops when the mobile keeper itself strikes the fixed magnetic pole).
In use, the electronic-control unit (ECU) of the engine determines the fuel quantity to be injected for each injector and, thus, by using the injection law, determines the corresponding actuation time for which the injector must be maintained open to deliver exactly the fuel quantity to be injected. Obviously, all injection-law errors (i.e., deviations of the estimated injection law that is stored in the electronic-control unit of the engine from the actual-injection law) directly affect the fuel quantity that is injected, determining a difference between the desired combustion and the actual combustion (with a potential increase of fuel consumption and a potential increase of generation of polluting substances).
Currently, a nominal-injection law of the injectors is stored in the electronic-control unit of the engine, but obviously the actual-injection law of each injector differs by a more or lesser extent from the nominal-injection law by effect of constructive tolerance and by effect of the deviation over time due to “aging” phenomena. In particular, an electromagnetic fuel injector displays a high dispersion of the injection features from injector to injector in a ballistic-operation area corresponding to short actuation times and, thus, small quantities of injected fuel. The manufacturers of controlled-ignition internal-combustion engines (i.e., working according to the “Otto” cycle) require electromagnetic fuel injectors capable of injecting small quantities of fuel (in the order of 1 milligram) with sufficient accuracy. Such a request is due to the observation that the generation of polluting substances during combustion can be reduced by fractioning fuel injection into several distinct injections. Consequently, it must be possible to use an electromagnetic fuel injector with high accuracy also in the ballistic area because only in the ballistic area can quantities of fuel in the order of 1 milligram be injected.
To attempt to reduce the injected-fuel quantity errors, it has been suggested to reduce the admissible maximum deviations between nominal-injection law and actual-injection law, particularly in the ballistic-operation area. However, such a request implies a considerable increase in the production costs of the injectors because it obliges to use more costly material, more sophisticated machining techniques [that are ultimately more costly (essentially because of the need to use more complex and accurate machine tools)], and greater controls during and at the end of construction (with a considerable increase in the number of incomplete parts or complete rejects). The matter is further complicated by the “fuel-injector aging” phenomena that determine a deviation of injection features over time.
Thus, there is a need in the related art for a method for determining the injection law of a fuel injector that it is free from the above-described drawbacks. More specifically, there is a need in the related art for such a method that is easy and cost-effective to implement in an existing electronic-control unit.